Figure 17.1 (a) Cytogenetics of a CML patient: Note the gain of material on chromosome 9 and the shortened chromosome 22 (22- or Ph chromosome). (b) Schematic representation of the t(9;22)(q34;q11) reciprocal translocation. Note the juxtaposition of BCR and ABL on the derivative chromosome that leads to a BCR-ABL fusion gene. An ABL-BCR fusion gene is formed on the derivative chromosome 9 but does not appear to play a role in the pathogenesis of CML. (c) Fluorescence in situ hybridization (FISH): The upper panel shows an interphase nucleus, and the lower panel a metaphase. FISH was done with the LSI bcr/abl ES probe (Vysis, Downer's Grove, IL) that detects the BCR-ABL fusion as well as the (red) signal on the derivative chromosome 9. (Courtesy of Christel Mueller, Department of Hematology, University of Leipzig, Germany)

b a c is crucial to establish the false-positive and false-negative rates of the probe set used, which vary significantly between different commercially available kits.21 The obvious disadvantage of FISH is that it screens only for the presence of the BCR-ABL translocation, and does not detect other cytogenetic abnormalities.

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